1. Field of the Invention
This invention relates to improvements in a method of controlling the cooling of a hot-rolled steel sheet after hot rolling and a system therefor.
2. Description of the Prior Art
In response to the aim toward reduced manufacturing cost of steel products, recently, there have been developed methods of producing hot-rolled steel sheets, including the technique of strengthening the transformed structure by the control of cooling after hot rolling as a measure of producing high strength steels in a hot rolled state using a steel material having a low alloy component value or values, the utilization of the technique of refining the crystal grains by the controlled rolling as a measure of simultaneously achieving both the high toughening and highly strengthening of the steels, the combined utilization of both the technique of strengthening the transformed structure and the technique of refining the crystal grains, and so on. There are proposed various techniques concerning the method of controlling the cooling and the conditions of cooling in these cases.
However, in most of these conventional methods, it is common to use the surface temperature of the hot-rolled steel sheet as being a material to be cooled as a control index of the conditions of cooling. In the cases where these methods are used, the following problems are posed:
(1) In measuring the temperature of a steel sheet in an actual system, a radiation pyrometer is normally used. However, it has been known that the radiation pyrometer is basically considered to be unsatisfactory in measuring accuracy, easily affected by the environment of measuring in particular, and easily subjected to measuring errors by steam and splashes of water and further due to the presence of cooling water, etc. remaining on the steel sheet for example, whereby, as a matter of course, there are presented such disadvantages that the positions of measuring the temperature are limited because the measurement of temperature cannot be performed in a cooling zone, the obtained information is not necessarily accurate information because merely the surface temperature is detected, and so on. When the above-described methods are adopted, there is a limit to the accuracy in controlling the conditions of cooling.
(2) As well known, when the steel is transformed from gamma phase to alpha phase, heating due to the transformation latent heat occurs, whereby the apparent specific heat is considerably varied depending on the progress of the transformation of the steel sheet, and, even if the steel sheet is cooled under the same cooling conditions, over-cooling, under-cooling or the like may easily occur depending on a delicate difference in the transformation properties, so that such disadvantages may be presented that a variability in the material quality is increased, evenness in form is deteriorated, or the like. It is well known that the transformation properties are complicatedly varied not only by the difference in the cooling conditions but also by the thermal strain history in the upstream process. In general, the aforesaid variations are usually occurring.
As a consequence, it is apparent that the above-described problems cannot be solved when there is adopted the method of controlling the cooling under the cooling conditions using the temperature as the control index as in the prior art. The most effective measure to obviate the above-described problems is to directly online-detect the behavior of transformation of the steel sheet and adopt a control method based on this information. As proposals concerning the above-described method for example, there are known Japanese Patent laid-Open No. 104754/1975 and Japanese Patent Publication No. 24017/1981.
However, each of the above-described proposals is a method of aiming at controlling the cooling conditions so that the transformation can always occur at a predetermined position when a variation in the transformation properties occurs at a position in the cooling zone, where the transformation occurs. Therefore, the method remains to the extent where slight improvements are applied to the method of using only the temperature as the index of control, as in the prior art. This is due to a deficiency in the means for detecting the behavior of the transformation and, for example, the detecting device proposed in Japanese Patent Laid-Open No. 104754/1975 can detect only the presence of occurrence of a gamma to alpha transformation. Further, in the case of Japanese Patent Publication No. 24017/1981, there is merely adopted an indirect means for detecting a phenomenon of heat-return during the transformation by a thermometer.
As a consequence, the behaviour of transformation of the steel cannot be satisfactorily grasped, therefore the controlling accuracy of the cooling conditions cannot be improved, thus posing the problem in the homogeneity of the material quality.
On the other hand, as the method wherein the gamma to alpha transformation rate is used as a control factor, the present applicants have proposed a method of producing the high strength steel excellent in flatness, featuring an accelerated cooling, in cooling after hot rolling a steel containing 0.005-0.5 wt % C, 0.05-2.0 wt % Si and 0.3-3.0 wt % Mn, in which a gamma to alpha transformation rate on the average from the start of cooling to the end of cooling falling within a range of 1-20 %/s is applied up to the time of the end of cooling where the gamma to alpha transformed fraction of the steel reaches a range of 60-100%, and after the time of the end of cooling, air cooling is applied. However, by this method, the transformation rate during the cooling was not strictly controllable as in the present invention.